Methods of imparting an embossed appearance to a cellular sheet include both chemical and mechanical embossing methods. U.S. Pat. No. 3,365,353 describes an improved method of chemically embossing a vinyl chloride resin cellular sheet material. This method incorporates a monomer compatible with a plasticized vinyl chloride into plastisol which is formed into a gelled resin sheet. A liquid containing a catalyst that will effect polymerization of the monomer in the formed vinyl chloride resin sheet is then applied in a predetermined pattern to one surface of the sheet and the resulting sheet heated to fuse the plasticized resin, decompose the blowing agent and to polymerize the monomer only in those areas where the catalyst has been applied, thereby obtaining a chemically embossed sheet material.
This method has a number of serious disadvantages. One major disadvantage is the requirement that the monomer be present throughout the entire plasticized vinyl chloride resin sheet. Typically, in the preparation of floor tile, only a minor portion of the tile is desired to have an embossed surface, such as the grout lines in an artificial stone pattern; that is, a depressed area, for example, 5 to 30%. Consequently, a major portion of the monomer in the plasticized vinyl chloride sheet is not contacted by the polymerization catalyst and not used, but results in a higher cost for the entire product. Another disadvantage is that the monomer selected must be wholly compatible with all components throughout the vinyl chloride resin sheet, since it is present throughout the entire body of the sheet, which compatibility limits the number of monomeric materials to be employed.
A further disadvantage of the described method is that the monomer by being blended into the plasticized vinyl chloride resin sheet formulation prior to the application or polymerization catalyst is subject to premature polymerization, either through the heat of the blending, mixing or gelation process or by the presence of the materials in the formulation, such as, for example, the metal salt stabilizers or other additives which may catalyze or lower the polymerization temperature of the monomer used. In addition, the presence of unpolymerized monomers in those areas of the sheet which have not been contacted with the polymerization catalyst applied in the printing ink is subject to change in physical properties on storage and aging or in temperature variations due to its polymerization tendency so that the resulting cellular chemically embossed sheet material may thus become in time more inflexible.
Furthermore, the employment of the polymerization catalyst in a liquid carrier is not an entirely efficient method of promoting the polymerization, since such polymerization depends upon the penetration of the polymerization catalyst into the sheet and the relatively uniform concentration of the catalyst during such penetration. Also, it is known that the use of materials in combination with certain metal salt stabilizers may tend to alter the decomposition temperature of the chemical blowing agent employed in a plasticized vinyl chloride resin sheet. Accordingly, where the monomer contains free hydroxyl groups, the incorporation of the monomer into the vinyl resin formulation may lower the decomposition temperature of the blowing agent used. Consequently, the blowing agent will decompose at a lower temperature and the polymerization catalyst used may correspondingly have a lower effective temperature in order to polymerize the monomer prior to substantial decomposition of the blowing agent.
Accordingly, my invention is directed to an improved chemical method of imparting an embossed appearance to a cellular sheet and to the embossed sheet products produced, which method and product are an improvement over the disclosure of U.S. Pat. No. 3,365,353.